PHILADELPHIA (CBS) — Only CBS 3 cameras were there as divers from the Philadelphia Police Marine Unit plunged into the freezing waters of the Schuylkill River, along Kelly Drive at mid-morning, training for ice rescue calls that may be coming as local rivers and waterways freeze over.

The divers, wearing special protective gear, made their way onto the river ice, dragging a special rescue sled. Secured by safety lines, and in touch with radios, divers went beneath the ice, after officers, using chain saws, cut holes.

Lt. Andrew Napoli told CBS 3 that the freeze-over gave his officers a unique opportunity to practice their lifesaving skills. Napoli cautioned that seeing people walking or playing on a frozen surface does not guarantee it is safe.

Divers told CBS 3’s Walt Hunter that beneath the ice, which is 6 inches thick in some spots, the river is flowing swiftly. They add that anyone breaking through would immediately be swept downstream, and, with the ice solid above them, they would be trapped.

The water was so cold that divers, even with protective gear, could only stay immersed for an hour, before they were affected. And, they added, the only thing anyone needs to know about frozen water safety is one simple rule: stay off the ice.